1. Field of the Invention
The present invention relates to improved methods and cement compositions for cementing subterranean zones penetrated by well bores.
2. Description of the Prior Art
Subterranean zones penetrated by well bores are commonly sealed by hydraulic cement compositions. For example, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein which supports and positions the pipe string in the well bore and seals the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also utilized in a variety of cementing operations such as sealing highly permeable zones or fractures in subterranean zones, plugging cracks or holes in pipe strings and the like.
Dispersing agents are commonly included in hydraulic cement compositions used for cementing subterranean zones. The dispersing agents reduce the rheologies of the cement compositions, i.e., the apparent viscosities and other properties of the cement compositions, as well as reducing the water content of the cement compositions. The reduction of the rheology of a cement composition allows the cement composition to be pumped with less friction pressure which utilizes less pump horsepower. In addition, the lower rheology often allows the cement composition to be pumped in turbulent flow. Turbulent flow characteristics are desirable when pumping cement compositions in wells in that drilling fluid is more efficiently removed from surfaces in the well bore as the drilling fluid is displaced by the cement composition being pumped. The inclusion of dispersing agents in cement compositions is also desirable in that the presence of the dispersing agents reduces the water required for preparation of the cement compositions. Cement compositions having reduced water content set into cement masses having improved compressive strength.
A number of dispersing agents have heretofore been utilized in well cement compositions. For example, certain organic acids such as gluconic acid and citric acid have been utilized as cement dispersing agents. However, such organic acids suffer from the disadvantage that they also function as cement composition set retarding agents which is often undesirable. That is, the presence of an organic acid dispersant in a cement composition prevents the cement composition from setting for a longer period of time which is often costly or otherwise detrimental. Other dispersing agents which are commonly used in well cement compositions include polynaphthalene sulfonate which is commercially available under the trade designation xe2x80x9cCFR-2(trademark)xe2x80x9d from Halliburton Energy Services, Inc. of Duncan, Okla.; the condensation product of formaldehyde, acetone and a sulfite which is commercially available under the trade designation xe2x80x9cCFR-3(trademark)xe2x80x9d from Halliburton Energy Services, Inc.; poly-B-naphthol sulfonate; polymelamine sulfonate; and many others. While the heretofore utilized dispersing agents have been used successfully, they generally require relatively high concentrations in well cement compositions and impart some set retardation to the cement compositions in which they are utilized. Further, some of the heretofore utilized dispersing agents also deteriorate the fluid loss control properties of cement composition fluid loss control agents.
Thus, there are continuing needs for improved methods of cementing and cement compositions wherein the cement compositions include a dispersing agent which can be utilized in smaller amounts, does not impart significant retardation properties to cement compositions and does not deteriorate the fluid loss control properties of fluid loss control agents.
The present invention provides improved methods and compositions for cementing subterranean zones penetrated by well bores which meet the needs described above and overcome the deficiencies of the prior art. The cement compositions of this invention are comprised of a hydraulic cement, sufficient water to form a slurry and a dispersing agent comprised of a tetrapolymer formed of methacrylic acid, methacrylate, methallyl sulfonic acid and ethoxylated acrylic acid.
The methods of the invention for cementing a subterranean zone penetrated by a well bore comprise the steps of providing a cement composition which comprises a hydraulic cement, sufficient water to form a slurry and a dispersing agent comprising a tetrapolymer formed of methacrylic acid, methacrylate, methallyl sulfonic acid and ethoxylated acrylic acid, placing the cement composition in the subterranean zone to be cemented and allowing the cement composition to set into an impermeable solid mass therein.
The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
As mentioned above, the well cement compositions of this invention are basically comprised of a hydraulic cement, sufficient water to form a slurry and a dispersing agent comprising a tetrapolymer formed of methacrylic acid, methacrylate, methallyl sulfonic acid and ethoxylated acrylic acid.
The methods of the present invention for cementing subterranean zones penetrated by a well bore are basically comprised of the following steps. A cement composition which comprises a hydraulic cement, sufficient water to form a slurry and a dispersing agent comprising a tetrapolymer formed of methacrylic acid, methacrylate, methallyl sulfonic acid and ethoxylated acrylic acid is provided. The cement composition is placed in a subterranean zone to be cemented and the cement composition is allowed to set into an impermeable solid mass therein.
The tetrapolymer dispersing agent includes methacrylic acid in an amount in the range of from about 50% to about 60% by weight of the tetrapolymer, methacrylate present in an amount of about 10% by weight of the tetrapolymer, methallyl sulfonic acid present in an amount of about 10% by weight of the tetrapolymer and ethoxylated acrylic acid present in an amount in the range of from about 20% to about 30% by weight of the tetrapolymer.
The ethoxylated acrylic acid in the tetrapolymer dispersing agent is ethoxylated within the range of from about 30 to about 120 moles of ethylene oxide. In a preferred tetrapolymer, the acrylic acid is ethoxylated with about 40 moles of ethylene oxide. In another preferred tetrapolymer, the acrylic acid is ethoxylated with about 100 moles of ethylene oxide.
A preferred tetrapolymer dispersing agent of this invention is comprised of methacrylic acid present in an amount of about 50% by weight of the tetrapolymer, methacrylate present in an amount of about 10% by weight of the tetrapolymer, methallyl sulfonic acid present in an amount of about 10% by weight of the tetrapolymer and acrylic acid ethoxylated with about 40 moles of ethylene oxide present in an amount of about 30% by weight of the tetrapolymer.
Another preferred tetrapolymer dispersing agent of this invention comprises methacrylic acid present in an amount of about 60% by weight of the tetrapolymer, methacrylate present in an amount of about 10% by weight of the tetrapolymer, methallyl sulfonic acid present in an amount of about 10% by weight of the tetrapolymer and acrylic acid ethoxylated with about 100 moles of ethylene oxide present in an amount of about 20% by weight of the tetrapolymer.
Examples of hydraulic cement which can be utilized in the cement compositions of this invention include, but are not limited to, Portland cements, slag cements, pozzolana cements, gypsum cements, aluminous cements and silica cements. Portland cements and their equivalents are generally preferred for use in accordance with the present invention. Portland cements of the types defined and described in API Specification For Materials And Testing For Well Cements, API Specification 10, 5th Edition, dated Jul. 1, 1990 of the American Petroleum Institute are particularly preferred.
The water in the cement compositions can be fresh water or salt water. The term xe2x80x9csalt waterxe2x80x9d is used herein to mean unsaturated salt solutions and saturated salt solutions including brines and seawater. The water is present in the cement compositions of this invention in amounts sufficient to form a pumpable slurry. Generally, the water is present in an amount in the range of from about 25% to about 100% by weight of hydraulic cement in the compositions.
The tetrapolymer dispersing agent is generally present in a cement compositions of this invention in an amount in the range of from about 0.01% to about 1% by weight of hydraulic cement therein, more preferably in an amount in the range of from about 0.01% to about 0.50%.
As mentioned above, the tetrapolymer dispersing agent of this invention is utilized in small quantities as compared to the quantities required by prior art dispersing agents, causes less set retardation and enhances the fluid loss control properties of fluid loss control agents included in the cement compositions.
An example of a fluid loss control agent, the properties of which are enhanced by the tetrapolymer dispersant is a copolymer of 2-acrylamido-2-methyl propane sulfonic acid and N,Nxe2x80x2-dimethylacrylamide which is commercially available under the trade designation xe2x80x9cHALADxe2x80x94344(trademark)xe2x80x9d by Halliburton Energy Services, Inc. of Duncan, Okla. The fluid loss control agent is generally present in the cement compositions in an amount in the range of from about 0.5% to about 2.0% by weight of the hydraulic cement therein.
A preferred method of this invention for cementing a subterranean zone penetrated by a well bore comprises the steps of: (a) providing a cement composition which comprises a hydraulic cement, sufficient water to form a slurry and a dispersing agent comprising a tetrapolymer formed of methacrylic acid, methacrylate, methallyl sulfonic acid and ethoxylated acrylic acid; (b) placing the cement composition in the subterranean zone to be cemented; and (c) allowing the cement composition to set into an impermeable solid mass therein.
A preferred composition of this invention comprises: a hydraulic cement; sufficient water to form a slurry; and a dispersing agent comprising a tetrapolymer formed of methacrylic acid, methacrylate, methallyl sulfonic acid and ethoxylated acrylic acid.